Spirogyra is propagated vegetatively by the breaking off of parts of the threads, which continue to grow as new plants. Resting-spores, which may remain dormant for a time, are formed by a process known as _conjugation_. Two threads lying side by side send out short projections, usually from all the cells of a long series (Fig. 266). The projections or processes from opposite cells grow toward each other, meet, and fuse, forming a connecting tube between the cells. The protoplasm, nucleus, and chlorophyll band of one cell now pass through this tube, and unite with the contents of the other cell. The entire mass then becomes surrounded by a thick cellulose wall, thus completing the _resting-spore_, or _zygospore_ (_z_, Fig. 266). * * * * * _Zygnema_ is an alga closely related to spirogyra and found in similar places. Its life history is practically the same, but it differs from spirogyra in having _two star-shaped chlorophyll bodies_ (Fig. 267) in each cell, instead of a chlorophyll-bearing spiral band. [Illustration: FIG. 267.—STRAND, OR FILAMENT OF ZYGNEMA, freed from its gelatinous covering. ] _Vaucheria_ is another alga common in shallow water and on damp soil. The thallus is much branched, but the threads are not divided by cross walls as in spirogyra. The plants are attached by means of colourless root-like organs which are much like the root-hairs of the higher plants: these are _rhizoids_. The chlorophyll is in the form of _grains scattered through the thread_. Vaucheria has a special mode of asexual reproduction by means of swimming spores or _swarm-spores_. These are formed singly in a short enlarged lateral branch known as the _sporangium_. When the sporangium bursts, the entire contents escape, forming a single large swarm-spore, which swims about by means of numerous lashes or cilia on its surface. The swarm-spores are so large that they can be seen with the naked eye. After swimming about for some time they come to rest and germinate, producing a new plant. [Illustration: FIG. 268.—THREAD OF VAUCHERIA WITH OÖGONIA AND ANTHERIDIA.]